Fasting for 24 hours could weaken the immune system

Fasting has previously been suggested as a useful approach for preventing or managing chronic diseases such as cancer, obesity and heart disease – but results from mouse studies show prolonged fasts may impair the immune system.

Filip Swirski at the Icahn School of Medicine at Mount Sinai in New York and his colleagues analysed blood and tissues samples from five mice without access to food for 24 hours. Blood tests comparing these mice with those given a regular diet showed that, on average, the fasting mice had less than 10 per cent of the number of monocytes – a type of white blood cell that helps fight infections and recruits other immune cells to treat injury.

“These are the cells that are really critical foot soldiers of the immune system,” says Swirski. The researchers found the decrease was due to monocytes retreating from the blood to the bone marrow, where they essentially hibernated, he says.

As a result, when the fasting mice were given food again, monocytes rushed back into their bloodstream. “Because there is this excess of monocytes that are hibernating and living in the bone marrow, they survive longer than they would otherwise,” says Swirski. “So, upon re-feeding, what we see is a surge of monocytes.” Compared with mice that continued to fast or those that never fasted, these mice had nearly four times as many monocytes in their blood, on average.

To understand how this affected immunity, he and his team injected 45 mice with a strain of bacteria that infects the lungs. Twenty-three of the mice fasted for 24 hours before the injection. The researchers then allowed them to access food.

After 72 hours, nearly 90 per cent of the mice that fasted died, while about 60 per cent of the mice that never fasted died. The mice on restricted diets also had greater levels of inflammation, suggesting that long periods of fasting impair immune responses.

However, Satchidananda Panda at the Salk Institute for Biological Studies in California says that most common fasting regimens for humans don’t last 24 hours. In fact, his own research has shown that a 15-hour fast improves immunity in mice.

Even so, Swirski says these findings are important for how we think about the duration and implications of fasting. “Like so many things in life, balance is important. So, what may be beneficial in one way could have an unanticipated negative impact in another,” he says.

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